It is frequently desirable to reinforce rubber articles (such as, tires, conveyor belts, power transmission belts, timing belts and hoses) by incorporating therein steel reinforcing elements. Pneumatic vehicle tires are often reinforced with cords prepared from brass-coated steel filaments. Such tire cords are frequently composed of high carbon steel or high carbon steel coated with a thin layer of brass. Such a tire cord can be a monofilament, but normally is prepared from several filaments that are stranded together. In most instances, depending upon the type of tire being reinforced, the strands of filaments are further cabled to form the tire cord. It is important for the steel alloy utilized in filaments for reinforcing elements to exhibit high strength and ductility as well as high fatigue resistance.
Transformation of the steel alloy into a filament suitable for reinforcing rubber articles involves multiple processing stages, including rough drawing, patenting, brass plating and fine drawing. The selected process to achieve a steel wire with defined characteristics can include many variations on those processing stages, including repeating the different stages.
Drawing of the wire reduces it from an original diameter to a smaller diameter by passing the wire through a conical die. Drawing of the wire increases the strength characteristics of the metal. Cold drawing can be done by using either wet or dry lubricants. Formation of a wire with desired properties may include multiple drawing steps both prior to and after patenting of the wire.
The object of patenting is to obtain a structure which combines high tensile strength with high ductility, and thus impart to the wire the ability to withstand a large reduction in area to produce the desired finished sizes possessing a combination of high tensile strength and good toughness. Patenting is normally conducted as a continuous process and typically consists of first heating the alloy to a temperature within the range of about 900° C. to about 1150° C. to form austenite, and then cooling at a rapid rate to a lower temperature at which transformation occurs which changes the crystal structure of ferrite from face centered cubic into pearlite, an eutectoid mixture of ferrite and cementite, which yields the desired mechanical properties. In many cases, while it is desired to form a fully pearlitic structure, additional phases can be present, such as undissolved carbides, pro-eutectoid cementite, and bainite.
For tire reinforcements, the continual goal is to increase the strength of the wire without a loss in ductility and fatigue resistance. In this quest for improved wire characteristics, the resulting wires have been characterized depending on the tensile strength by using different identifiers such as high tensile, super tensile, ultra tensile strength, and mega tensile wherein each wire strength is defined by a minimum tensile strength.